Conventional geothermal heat pumps (GHPs) have the potential to provide significant energy savings over typical air-source heat pumps or typical furnaces with air conditioners. GHPs are a proven technology and savings in the range of 30 to 60 percent have been documented, but the greatest barrier to acceptance of GHPs in the marketplace is their high initial cost and long payback period.
This invention uses nanofluid in the circulation loop of a GHP that will increase the heat transfer; thereby reducing the installation cost and reduce the operating cost of GNPs. Improved heat transfer will lower the installation cost because the circulation loop can be smaller and the pumping cost will also be less. Nanofluids are not new in fact human blood is a nanofluid. The invention introduces nanoparticles into the circulation loop (propylene glycol or a heat transfer oil) used in today's systems. Prior work has already shown that using nanoparticles are an excellent method to improve thermal conductivity in water, ethylene glycol, engine oil and refrigerant applications. Improved thermal conductivity enhancement of 40% or more has been reported with only a 0.10% nanoparticles concentration.
The most common method to increasing the heat transfer rate in a cycle is to use extended heat transfer surfaces for exchanging heat with a heat transfer fluid. This approach produces an undesirable increase in the sizes of the heat exchange device and a larger circulation loop. In ground source heat pumps the main heat exchanger is the bore field or pond. In addition, the inherent poor thermodynamic properties of conventional heat transfer fluid (ethylene or propylene glycol) limits the amount of heat transfers. Therefore, there is a need to develop advanced cooling techniques and innovative heat transfer fluid with better heat transfer performance than those presently available.
It is well known that metallic solids possess order of magnitude higher thermal conductivity than conventional heat transfer fluids. For example the thermal conductivity of copper is 3000 times greater than engine oil. In the past, researchers have tried to increase the thermal conductivity of base fluids by suspending micro or large sized solid particles into the fluid, because the thermal conductivity of solids such as copper is so much higher than that of liquids. Prior researchers expected that the metallic particles would significantly increase the heat transfer. Unfortunately, when this has been tried, large size particles follow Maxwell's theory in that they lack stability and settle out of the liquid. The suspension also causes additional flow resistance and possible erosion problems which are negative effects of using a mixture of a base liquid with suspended large metallic particles.
Nanotechnology provides new opportunities to produce material with an average particle size below 100 nm (nanometer). These nanoparticles do not follow Maxwell's theory and have a much large relative surface area as compared to conventional particles. Unlike suspension discussed above, nanoparticles not only improve heat transfer, they also may reduce flow friction, improve the kinetics of the heat transfer and can be made to remain in a stable suspension for long periods of time.
It has been documented that the addition of nanoparticles has remarkably enhanced the thermal conductivity of the base liquid. These nanofluids are quite different from conventional two-phase flow mixtures discussed earlier. It has been demonstrated that nanoparticles can improved thermal conductivity by a 2 to 3 fold increase. In addition, nanoparticles resist sedimentation, as compared to larger particles, due to Brownian motion and inter-particle forces.
The focus of the patent is on ground source heat pumps (GHP); yet, the proposed technique can also be used as a cost-effective method for improving absorption cooling, engine oil cooling, water and glycol cooling systems, and all water systems HVAC (Heating, Ventilation, Air Conditioning). This invention uses propylene glycol, ethylene glycol, or food grade heat transfer oil as the fluid. These nanofluids can be used in any other heat transfer application such as solar collectors, solar concentrators and other heat transfer applications. In the preferred embodiment the food grade heat transfer oil is Paratherm LR™. Paratherm LR™ is an aliphatic-hydrocarbon based heat transfer fluid. This invention is not limited to Paratherm LR™ but rather includes all heat transfer oil that are aliphatic-hydrocarbon based. This invention covers ethylene glycol as well as propylene glycol. Environmentally friendly propylene glycol is always a better choice of fluid but this invention will work for both ethylene and propylene glycols.
Dr. Steven Choi from US Department of Energy's (DOE's) Argonne Labs is usually credited for inventing nanofluids [U.S. Pat. No. 6,221,275B1]. Dr. Choi documented the increase in thermal conductivity. Over time it has been recognized that thermal conductivity is not the best figure of merit to evaluate nanofluids because the nanoparticles tend to increase the viscosity of the nanofluid. Increased viscosity can increase pumping cost and can reduce heat transfer because the boundary layer will increase. Argonne Labs is patenting a new nanofluid and describes a testing apparatus to measure heat transfer properties [Pub. No. US2011/0001081 A1]. Their apparatus measures more properties than thermal conductivity. It has been recognized that thermal conductivity of the nanofluid is not a good indicator of heat transfer performance of a fluid. The Argonne lab device does not have an isothermal cold tank and does not model GHPs. Argonne labs now recommends using a new figure of merit for nanofluids called the Mouromtseff (Mo) number which is a function of the density, viscosity, thermal conductivity and specific heat and not thermal conductivity alone.
      M    ⁢                  ⁢    o    =            (                        ρ          0.8                *                  k          0.67                *                  cp          0.33                    )              μ      0.46      where ρ is the density, k is the thermal conductivity, Cp is the specific heat and μ is the dynamic viscosity.
The nanofluids covered by this invention have a high Mo figure of merit for heat transfer.
The typical GHP consists of a heat pump, circulating pump, heat exchanger, bore field or pond heat exchanger, and a circulating fluid. This invention improves that heat transfer of the circulating fluid in the bore field or pond by providing a nanofluid that reduces energy consumption and does not have the harmful effects of ethylene glycol. Ethylene glycol is a hazardous material that might be consumed by people should the ethylene glycol get into the drinking water because of a leak in the bore field or pond or through leaks in the fittings and seals of the circulating loop system.